The present invention concerns a variable focal lens device comprising a variable power lens group and a focusing lens group for use in cameras, which calculates an amount of change in the focal length resulted upon positioning the variable power lens group to obtain a desired magnifying power and conducts a positional compensation for the focusing lens group automatically depending on the amount of change.
Zoom lens system can provide a satisfactory operability because it does not require to focus on an object on every zooming operation since the zooming operation causes no positional image deviation (so-called shift of focus or out of focus). On the other hand, the focusing operation in a single lens reflex type for example, needs skillful technic since the view is less bright as compared with that of the single focus lens. Since the foregoing problem has been dissolved with the recent development in the automatic focus control in cameras, users can concentrate their attention only to the decision of a composition of a picture, which improves the operability significantly.
Focusing in the zoom lens is generally conducted by moving a focusing lens group of a variable power optical system. The zoom lens has an advantage that the amount of the movement of the focusing lens group is substantially constant with an identical object distance for the entire zoom zone (this is referred to as "equal amount movement" hereinafter), and accordingly, an advantage that the graduation for the object distance may not necessarily be changed depending on the zooming. However, in the zoom lens of an inner focusing system or rear focusing system, there has been a problem that the lens constitution becomes complicated, and in addition, the moving amount (delivery amount) of the focusing lens group is increased unnecessarily on the wide angle side if the optical design is conducted under the condition of realizing the equal amount movement. Further, this also results in a problem of enlarging the outer diameter of the lens and increasing the weight of the lens and the lens barrel.
The present invention has been made in view of the foregoing situations and object thereof is to provide a variable focal lens system for use in cameras, which is simple and compact in the mechanism, since it does not require to make the amount of movement of the focusing lens group identical with that of the variable power lens group on the same optical axis.
The foregoing object of the present invention can be attained by a variable focal lens device having an optical system comprising a variable power lens group and a focusing lens group disposed on one identical optical axis in which a deviation of a focusing position is resulted with an identical object when the focusing lens group is set at a predetermined focusing position from a nearest position to an infinite position on the optical axis corresponding to an object distance from a nearest distance to an infinite distance and then moving said variable power lens group along the optical axis thereby renewing an entire system focal length of said variable power optical system from a first focal length to a second focal length wherein said device comprises;
a focal length detection means for detecting said entire system focal length,
a focusing lens group position detection means for detecting a position of said focusing lens group on said optical axis,
a maximum delivery amount calculation means for receiving an output from said focal length detection means and calculating a delivery amount of said focusing lens group from said infinite position to said nearest position for said focal length,
a proportional constant calculation means for receiving outputs from said maximum delivery amount calculation means and from said focusing lens group position detection means respectively and calculating a ratio between said outputs,
a focusing compensation calculation means for receiving outputs from said proportional constant calculation means, from said maximum delivery amount calculation means and from said focusing lens group position detection means respectively, and calculating an amount of deviation of the focusing position from the focusing position resulted from the renewal of said entire system focal length as a compensation value,
a focusing drive means for driving said focusing lens group,
a moving amount monitor means for generating a signal corresponding to the moving amount of said focusing lens group,
a focusing control means for receiving outputs from said moving amount monitor means and from said focusing compensation calculation means respectively and driving said focusing lens group to said focusing position,
a variable power drive means for driving the variable power lens group, and
a variable power control means for receiving a start signal from a start means and controlling said variable power drive means, the deviation of the focusing position due to the renewal of the entire system focal length of said optical system being automatically compensated.
The foregoing object of the present invention can be attained also by a variable focal lens device having an optical system comprising a variable power lens group and a focusing lens group disposed on an identical optical axis in which a deviation of a focusing position is resulted with an identical object when the focusing lens group is set to a predetermined focusing position from a nearest position to an infinite position on the optical axis corresponding to an object distance from a nearest distance to an infinite distance and then moving said variable power lens group thereby renewing an entire system focal length of the optical system from a first focal length to a second focal distance, wherein the device comprises;
a focal point detection means for detecting a focal point adjusting state of said focusing lens group to said object,
a focusing control means for driving the focusing lens group so as to focus on the object based on an output from the focal point detection means,
a variable power lens group position detection means for detecting a position of said variable power lens group on said optical axis corresponding to said entire system focal length,
a focusing lens group position detection means for detecting a position of said focusing lens group on said optical axis corresponding to said object distance,
a central position calculation means for calculating a central position between said nearest position and said infinite position at the focal length thereby outputting a signal corresponding to the central position when detecting a focal point by said focal point detection means is impossible, and
a judging means for judging a relative relationship between the position of said focusing lens group and said central position based on an output from said focusing lens group position detection means and an output from said central position calculation means, thereby instructing said focusing control means to drive said focusing lens group toward a near distance side in case said focusing lens group is situated on a far distance side with respect to said central position, while toward a far distance side in case said focusing lens group is situated on a near distance side with respect to said central position.